Isn't SVS making the positive claim?
Claim: SVS is better than public schools in income-disadvantaged settings.
Claim: SVS is better than public schools when students are not parent-selected.
If there is no evidence, then these claims are undisproved (that is, not proved and
also not disproved). Given the nature of SVS, I doubt there is much evidence on
either of these claims. In fact I can't even think of a good way of testing them.
But the claims might be tested someday. Until then we just don't know.
Some guy who hated public school,
And writes interesting (hopefully) email.